Estimating Demographics Of The Nile Crocodile (Crocodylus niloticus Laurenti) In The Panhandle Region Of The Okavango Delta, Botswana

The effective conservation and management of world crocodilians often involves a range of management interventions (for example, strict protection to harvesting), depending on the status of the species, their habitats and the national context within which crocodilians and people interact. The ability to quantify the relative and ⁄ or absolute abundance of a population, and population structure in terms of juveniles versus adults, is fundamental to management.

Throughout their range in Africa, widespread crocodile eradication programs were implemented in the 1930s and continued into the 1960s with the added incentive of selling hides (Cott, 1961; Parker & Watson, 1970). This slaughter abated when international trade came under control of the Convention on International Trade in Endangered Species of Wild Fauna and Flora in 1975. Today, Nile crocodiles are on Appendix II in Botswana, Ethiopia, Kenya, Malawi, Mozambique, South Africa, Tanzania, Zambia and Zimbabwe (CITES, 2007). In the other 33 range states, they are listed as CITES Appendix I. The main factors reducing Nile crocodile populations have been unsustainable hunting for skins, meat and pest control, and both habitat loss and pollution (Cott, 1961; Abercrombie, 1978; Thorbjarnarson, 1992). The Crocodile Specialist Group of the IUCN has recommended different management strategies to sustain different national populations of Nile crocodiles, depending on their current status (Thorbjarnarson, 1992).

The Nile crocodile population in the Okavango Delta has undergone three periods of human-induced decline over the last century. In 1957, the Department of Wildlife and National Parks (DWNP) allowed a quota of 2000 animals per year for the hide-trade, to each of two concessionaries. Between 1957 and 1969, although 40,000–50,000 crocodiles were thought to have been killed ⁄ traded (Taylor, 1973; Pooley, 1982), c. 80,000 crocodiles may actually have been removed because some estimates indicate that only 50% of crocodiles shot were recovered (Taylor, 1973).

In 1973, the DWNP set a quota of 500 animals per year for the Botswana Game Industries to resume hide-hunting (Graham, Simbotwe & Hutton, 1992). This quota was filled in 1973, but only 440 crocodiles were shot in 1974 and the venture was thus regarded as uneconomic and disbanded (Taylor, 1973). After a decade of no exploitation, crocodile farmers removed 1053 live adults for captive breeding and 14,000 eggs from the system in the period 1983–1988 for commercial use in ranching operations. According to an aerial nesting survey conducted by DWNP in 1987, this led to an estimated 50% reduction in the breeding population (10,000 individuals) (Simbotwe & Matlhare, 1987). Subsequently, from 2004 to 2006, a local crocodile farm harvested 2000 eggs per year. In addition to this, disturbance by boat motors (Mbaiwa, 2002), fires and destruction of nests and eggs by fishermen (Shacks, 2006), habitat loss, crocodile ⁄ human conflict (Thomas, 2006) and pollution are contributing to the crocodiles decline in the Okavango.